Process for producing photoconductive materials

ABSTRACT

A PROCESS FOR PRODUCING A PHOTOCONDUCTIVE POWDER ACCORDING TO THE VALENCE CONTROL METHOD, IN WHICH A PHOTOCONDUCTIVE INORGANIC POWDER IS FIRED IN THE PRESENCE OF AN ACTIVATOR, A COACTIVATOR AND A FLUX, CHARACTERIZED IN THAT THE FIRING IS EFFECTED IN THE PRESENCE OF A DISPERSANT HAVING SUCH PROPERTY THAT IT CONTACTS WITH THE PHOTOCONDUCTIVE POWDER, WITHOUT MELTING AT THE FIRING TEMPERATURE, AND, DOES NOT FUSE NOR AGGLOMERATE THE PHOTOCONDUCTIVE POWDER. THE PROCESS IS PARTICULARLY APPLICABLE TO PHOTOCONDUCTIVE POWDER CONTANING CADMIUM SULFIDE, CADMIUM SELENIDE OR CADMIUM SULFOSELENIDE.

United States Patent 3,743,609 PROCESS FOR PRODUCING PHOTO- CONDUCTIVEMATERIALS Masao Hirata and Noriyoshi Tarumi, Tokyo, and Masayuki Sato,Ohtsuki, Japan, assignors to Konishiroku Photo Industry Co., Ltd.,Tokyo, Japan No Drawing. Filed Dec. 23, 1971, Ser. No. 211,726 Claimspriority, application Japan, Dec. 26, 1970, 45/ 119,052 Int. Cl. H01c7/08; G03g 5/08 US. Cl. 252501 1 Claim ABSTRACT OF THE DISCLOSURE Thisinvention relates to a process for producing photoconductive finepowders of cadmium sulfide, cadmium selenide, cadmium sulfoselenide,etc.

' As is well known, photoconductive powders have been used in the formof resin-bound layers, sintered mass or resin-coated dusts, for a devicefor changing signals of light or radiation into electric signals whichare useful for electrophotography, e.g. image converters, imageintensifiers, photocells, etc.

In the case of a cadmium sulfide type photoconductive powder, ingeneral, a pure cadmium sulfide powder is mixed with an acceptor-formingactivator such as copper or silver, a donor-forming coactivator such ashalide, and a flux such as cadmium chloride, sodium chloride or zincchloride, and then the resulting mixture is fired at a temperature abovethe melting point of the flux to carry out the atomic valence control,whereby a photoconductive powder having various characteristics can beobtained. Ordinarily, however, several to several tencadmium sulfideparticles fuse to one another at the time of firing to form aphotoconductive powder of about several microns to 30 microns inparticle size. The degree of particle size of such photoconductivepowder frequently brings about a great problem. For example, in forminga photosensitive layer, the degree of particle size influences theuniformity of the photosensitive layer. That is, if the particle size islarge, the surface smoothness of the photosensitive layer is injured.Further, in forming a coating film by use of a dispersion of the powderin a resinous hinder, the dispersion should be made high in viscosity inorder to lower the sedimentation velocity of the particles, with theresult that there are brought about such drawbacks that not only thecoating operation is extremely difiicult but also bubbles becomeentrained in the resulting photosensitive layer.

The above-mentioned drawbacks due to large particle size brings aboutdegradation of image quality in the case of image-forming photosensitivemembers for use in electrophotography, image converters, imageintensifiers, etc.

When applied to an electrophotographic photosensitive member, a finephotoconductive powder gives such advantages that the photosensitivemember is greatly decreased in fog, increased in contact portions ofindividual particles, and increased in electric resistance. Further, theuniformity of the photosensitive layer due to incor- 3,743,609 PatentedJuly 3, 1973 poration of fine photoconductive powder has such meritsthat the damage of photoelectric means due to local eddy currents can beprevented and the distance between electrodes can be made small.

For the production of a photoconductive powder, which is finer than thatproduced in the above-mentioned manner, there may be thought of thefollowing processes:

(1) The prior art photoconductive powder is further pulverized.

(2) In the step of producing the photoconductive powder, the firing iseflfected by use of no or smaller amount of flux to inhibit the growthof crystals.

(3) In the step of producing the photoconductive powder, the firing iseffected at below the melting point of the flux to inhibit the growth ofcrystals.

According to process (1), however, the resulting crystals are far lowerin photosensitivity than the prior art photoconductive crystals.Further, according to processes (2) and (3), sufiicient activationcannot be accomplished to make optional valence control impossible, withthe result that no high sensitivity material can be obtained.

As a specific process, there has also been known the following process:

(4) A process for producing a high sensitivity fine photoconductivepowder having a particle size of less than 5 microns by mixing a cadmiumsulfide powder with suitable amounts of an activator, a coactivator andpure water, and treating the resulting mixture at elevated temperatureand pressure to recrystallize the cadmium sulfide.

This process, however, is required to be carried out at high temperatureand pressure for a long period of time (about 50 hours), and hence islow in practicality.

The present invention is a process for producing a fine photoconductivepowder by firing a mixture comprising a powder of cadmium sulfide,cadmium selenide, cadmium sulfoselenide or the like (starting material)and suitable amounts of an activator (a halide, sulfate or nitrate ofgold, silver or copper), a coactivator (a halide such as ammoniumchloride or the like, or a compound of a trivalent metal such asaluminum, gallium, indium or the like), a flux (a halide such as cadmiumchloride, zinc chloride, sodium chloride, potassium chloride or thelike), and a dispersant (sodium iodide, potassium iodide, sodiumbromide, potassium bromide, sodium chloride, potassium chloride, sodiumsulfate, calcium carbonate, sodium carbonate, calcium oxide or thelike), whereby the degree of size of the growing crystals is controlledby the starting powder.

The activator used herein is a chemical which forms the acceptor levelin the crystals of the starting cadmium sulfide or the like. Thecoactivator is a chemical which forms the donor level in the crystals ofthe starting cadmium sulfide or the like. Both the activator and thecoactivator are to provide desired properties of the photoconductor. Theflux is a chemical which has such property that in the firing step itmelts at the firing temperature to fuse the starting powder. It shouldbe understood that although the functions of the activator, coactivatorand flux have been mentioned as above, sometimes a single chemical canperform two or three of the functions. The dispersant is a chemicalwhich has such property that in the firing step, it neither melts at thefiring temperature nor fuses the starting powder. The flux and thedispersant are distinguished from each other depending on the firingtemperature. That is, depending on the firing temperature, there are thecase where a substance is used as the flux and the case where the saidsubstance is used as the dispersant. For example, sodium chloride whichhas a melting point of 800 C. is used as the flux when the firingtemperature is more than 800 C., but may be used as the dispersant whenthe firing temperature is too low to melt the sodium chloride.

The dispersant used in the present invention has such property asmentioned above. One or two or more of such compounds may be used. Thedispersant serves to divide the activated starting material intosuitable fine units to inhibit the fusion and agglomeration of thestarting particles. Further, the particle size of the resultingphotoconductive powder is affected by the mixing ratio of the dispersantto the starting material and by the particle size of the dispersant. Byselection of the dispersant, the particle size of the resultingphotoconductive powder can suitably be controlled. Ordinarily, thedispersant is used in excess of the starting material.

It is desirable to use as the dispersant a substance having suchproperties that it is high in purity, has no chemical interaction withthe starting photoconductive powder, is higher in melting point than theflux used, does not melt at the firing temperature, can disperse thephotoconductive powder, and is Water-soluble. For example, in casecadmium chloride is used as the flux, sodium chloride, potassiumchloride, sodium bromide, potassium bromide, sodium iodide, potassiumiodide, cadmium sulfate, zinc sulfate, sodium sulfate or potassiumsulfate is preferable as the dispersant for the reasons that saidcompound is higher in melting point and lower in solubility in alcoholsthan cadmium chloride and hence can uniformly be dispersed by use ofalcohol, and that it is water-soluble and hence can easily be separated,after firing, from the photoconductive powder by water-washing. Thefiring temperature in the present invention may be the firingtemperature adopted in the known valence control method, and shouldsuitably be selected in consideration of the kind of the startingmaterial and the desired characteristics of the resultingphotoconductive material.

The present invention is illustrated in further detail below withreference to examples.

EXAMPLE 1 A mixture comprising 175 g. of high purity cadmium sulfide of0.5 to 1 micron in particle size, 50 g. of cadmium chloride as a flux,6.4 g. of ammonium chloride as a coactivator, 15 cc. of an aqueoussolution of mole/ cc. of copper chloride as an activator and 140 cc. ofpure water was pulverized for 6 hours in an agate-made ball mill.Thereafter, the mixture was transferred to an evaporating dish and driedat 140 C. for about hours. To the dried mixture were added 600 g. ofsodium chloride as a dispersant and 400 cc. of absolute alcohol. Theresulting mixture was pulverized for 6 hours in an agate-made ball mill.Thereafter, the mixture was placed in an evapo rating dish and thendried at 120 C. for about 10 hours. The dried mixture was ground to thesize of millet grains, and the resulting grains were charged into aquartz tube and then fired by use of an electric furnace in an air atmosphere at 590 C. for 15 minutes. After cooling to room temperature,the resulting brown fired substance was dipped in pure water,water-washed by decantation and then dried. The water-washing wasterminated after repeating the decantation about 20 times to confirmthat no chlorine ion had been detected in the waste liquid. The firedsubstance after drying was a brown crystalline powder of about 1 micronin particle size and showed photoconductivity. The particle sizedistribution of the thus obtained cadmium sulfide was measured by use ofan optical microscope, and the percentage of the number of particlesvarying in size is shown in Table 1.

In this example, absolute alcohol is used for efiective dispersion, butit is not always required to be used and may be replaced by anotherorganic solvent. However, it is effective to use a solvent which is lowin solubility for the dispersant.

EXAMPLE 2 Example 1 was repeated, except that an aqueous solution of 10-mole/ cc. of AgNO was used as the activator, KI as the flux and CdSO asthe dispersant, and the firing temperature was 700 C. The resultsobtained were as set forth in Table 1.

EXAMPLE 3 Example 1 was repeated, except that the amounts of thestarting material, flux, activator, coactivator and dispersant used werevaried as shown in Table 1. The results obtained were as set forth inTable 1.

EXAMPLE 4 Example 1 was repeated, except that cadium selenide was usedin place of the starting cadmium sulfide, and the amounts of individualcompounds used were varied as shown in Table 1. The results obtainedwere as set forth in Table 1.

EXAMPLE 5 Example 1 was repeated, except that the starting cadmiumsulfide Was replaced by cadmium sulfoselenide and the amounts of theindividual components used were varied as shown in Table 1. The resultsobtained were as set forth in Table 1.

TABLE 1 Example Comparative Example Starting material Cris Cds Cds GdsnGdSSe CdS O CdS. Amount (g.) 17 1 17 48 180 180 180 180. Flux CdClz KICdCh CdCLs CdCh C(1C12, CdClz 0111011 5 2 14.0. 5 2 rho. 5/2 1120 5 2filo. 5 2 H2O. 5 2 rho. 5 2 rho. Amount (g.) 50 50 5.6- 50 27.6. 25 28.Coactivatm' NHJ NHr NHl NH Cl NH4O NH Cl NH C1 NH C1. Amount (g.) 6.4-.2.0... 6.4"--- 0.3-- 6.4-. 1.2... 1. 1.3. Activator Aqueous AqueousAqueous Aqueous Aqueous Aqueous Aqueous Aqueous solution stglutionsglution solution solution sglutlon stzlutiou solution 0 o o o o o 10-moi/cc. 10- moi/cc. 10- mol/cc. 10- mol/ec. 10- molloe. 10- moi/cc. 10-mol/eo. 10- moi/cc. of CuClz. of Ag(NO3). of CuClz. of 011012. of CuCl:.oi CuClz. of 011012. of Ouch. Amount (cc.) 1% .5. 3.0-- 15 2.2.- 1.2"---0.12. H20. H20 H20 H20 H20 H20 H20 H20 H10. Amount (cc.)- 14o 1'40 140an 140 140 140 140, Dispersant NaCl CdSO NaOl NaCl NaCl. NaOl- NaCL.Amount (g.) am 200 a so 1 no 400 Particle size distribution (percent):

0.5-1 '4 6.- 11 1-9.. 93 29 or 83 95 2-5 9 57 a 4 2 4 3, 5-1 m. a 24 2333, 102nu 2Q 39 19. 20-30 29 26 29. 3040.. 14 1';

In Table 1, there are also shown, for comparison, the results obtainedby repeating Example 1, except that the dispersant was not used and theamounts of the individual components used were varied as shown in Table1.

In order to enhance the sensitivity of the activated cadthephotoconductive powders according to the comparative examples, and thephotosensitivities thereof are lowered.

In the case of the photoconductive powders according to the comparativeexamples, the voltage at which the mium sulfide powder obtained in eachof the above-men- 5 cells had been damaged was 600 to 700 volts, whereastioned examples, there may be eifected the following treatin the case ofthe photoconductive powders according to ments. the present invention,said voltage was more than 2,000 The crystalline powder afterwater-washing is mixed volts. This is considered ascribable to the factsthat the with 0.05 g. of cadmium chloride and 1.0 g. of ammo- 10 voltageapplied had been consumed by a barrier layer nium chloride. Theresulting mixture is dried at 140 C. derived from resistivity contactamong the particles arfor about 10 hours. Subsequently, the driedmixture is ranged between the electrodes, and that the uniformity ofcharged into a quartz glass tube and fired by use of an thephotosensitive layer had been increased due to the electric furnace inan air atmosphere at 590 C. for use of finer photosensitive particles.minutes. The fired mixture, which has been taken out of 15 On the otherhand, 5 g. of the photoconductive powthe furnace after firing, is insuch a state that crystalline der prepared in Example 1 was subjected tosensitivityparticles, which are susbtantially in particle size with theincreasing treatment, and then dispersed in a mixture particles beforefiring (about 1 micron), have slightly comprising 2 g. of alkyd resinand 2 g. of xylene. The readhered to one another, and can be easilyground. After sulting dispersion was coated on an aluminum plate andgrinding in an agate-made mortar, the said soft fired mixthen dried andcured to prepare an electrophotographic ture is packed in a quartz glasstube, and fired first in a light-sensitive plate having a sensitivelayer of about 70 hydrogen sulfide-nitrogen gas atmosphere at 500 C. formicrons in thickness. The sensitive layer of said light-sensi- 10minutes and then in a vacuum atmosphere at 500 C. tive plate waselectrically charged on the surface by use for 10 minutes. Subsequetly,the first mixture is cooled of a corona discharge electrode, to whichhad been applied to 100 C. in said vacuum atmosphere and then dried in avoltage of 6 kv., and then subjected to imagewise exa desiccator. posurefor 0.2 second so that the bright portion became As is clear from Table1, the photoconductive powder luxes to form a static latent image.Subsequently, the produced according to the present invention iscomposed latent image was developed with a toner according to an of fineparticles having a particle size within the range ordinary procedure andthen transferred to a high quality from 1 to 5 microns, and not only theparticle size dis- 30 paper, whereby a clear image free from fog wasobtained. tribution thereof can freely be controlled by varying theThus, even when used as an electrophotographic lightblending amounts ofthe individual components (refer to sensitive layer, the photoconductivepowder obtained ac- Example 2) but also the fine particles aresubstantially cording to the present invention gives favorable results.in the form of spheres. Accordingly, when the photocon- What we claimis: ductive powder is formed into a photosensitive layer, the 1. Aprocess for producing a photoconductive powder charging rate of thephotosensitive material can be made according to the valence controlmethod comprising firing greater. a photoconductive material selectedfrom the group con- In contrast thereto, the photoconductive powderprosisting of cadmium sulfide, cadmium selenide and cadmium ducedaccording to any of the known processes (refer to sulfoselenide in thepresence of an activator, a coactivator, comparative examples) is notonly composed mainly of a flux, and a particulate dispersant selectedfrom the group particles having a particle size within the range from 10consisting of sodium iodide, potassium iodide, sodium broto 30 micronsbut also contains particles having a parmide, potassium bromide, sodiumchloride, potassium chloticle size of more than 30 microns, and shows awide parride, sodium sulfate, calcium carbonate, sodium carbonate ticlesize distribution. Moreover, the particles are comand calcium oxide at atemperature which is below the posed of spherical, rod-shaped andL-shaped particles. melting point of the dispersant but sutlicient tomelt the The photoconductive powders prepared according to flux andintroduce the activator and coactivator into the the aforesaid examplesand comparative examples were photoconductive material, wherein theamount by weight subjected to sensitivity-increasing treatment. 5 gramsof of dispersant is in excess of that of the photoconductive each of thethus treated photoconductive powders was dis material.

TABLE 2 Comparative Example Example Applied voltage (V) 100 100 100 100100 Ampere (A) after allowing to stand in the dark for 15 min zxm-213x10 2.8X10- 1.1 10- lxio- Ampere (A) after irradiation of 10 luxlightforlmin 1.5x10- 6.2X10- 4.2 10 4.s 10 1.5 10- persed in a mixturecomprising 2 g. of an alkyd resin References Cited (J-555 produced byDai-Nippon Ink. Co.) and 2 g. of UNITED STATES PATENTS xylene. Theresulting dispersion was coated on comb type electrodes (electrodedistance 1 mm., length 150 mm.), 2,856,878 12/1958 BnggF et a1 252-501 Xwhich had been prepared by the vacuum deposition of 3,238,150 3/1966Behrmger et a1 96 1-5 X aluminum onto insulating polyester films, andthen dried 3,598,760 8/1971 Nakamura et a1 961'5 X and cured.Subsequently, a voltage of 100 v. was applied 2,986,534 5 Banner 252-401across the electrodes, and the amperes in the dark and the 3,037,941 9/1962 Ranby et a1 bright were measured by means of a microammeter. The3593643 7/1971 results obtained were as set forth in Table 2 above.3,694,201 9/1972 Behrmgel' 252-501 X From Table 2, it is understood thatwhen a voltage is applied to comb type photoconductive cells preparedCHARLES E'VAN HORNPnmary Exammer by use of the photoconductive powdersaccording to the U S Cl X R present invention, the dark and brightcurrents of the cells are far lower than in the case of cells preparedby use of

